In recent years, a power generation technique utilizing natural energy such as solar light and a smart grid technique efficiently operating power systems making full use of information techniques have attracted attention. For the power generation utilizing the natural energy, it is difficult to manage a power generation amount to be constant due to the influence of the weather and the like. Therefore, in a power system in which a large number of natural energy-based power generation systems are connected, there is a fear that a system accident occurs due to unbalance of supply and demand and a power failure occurs in the worst case. For the purpose of avoiding such situation, for example, a power storage module including, as a component, a unit battery of a secondary battery such as a lithium-ion battery (also referred to as single battery or cell. It will be appropriately referred to as battery cell in following description) or a battery block is used. Some of such power storage modules are capable of performing megawatt output.
The power storage module configures the battery block by connecting a plurality of (e.g. 4, 8, 10) battery cells in parallel. The series connection of a large number of battery blocks is housed in an outer case for configuring the power storage module. In addition, in the case where a large number of battery cells are used for generating a large output, a battery system in which a plurality of power storage modules are connected in series and a common control apparatus is provided for the plurality of power storage modules is known. Each power storage module includes a module controller and is configured to perform communication between the module controller and the control apparatus via a communication means.
In the case where a plurality of battery cells are used, even when one of the plurality of battery cells reaches a lower limit voltage during discharging, the other battery cells may not still reach the lower limit voltage due to a difference or the like in self-discharge of the battery cells. When the battery cell is charged again from this state, some battery cells may not be fully charged and a problem in that those battery cells cannot work sufficiently arises. The plurality of battery cells are charge/discharged from the state in which there are variations in the charging state among the plurality of battery cells connected in series, the same current flows through the plurality of battery cells connected in series. Therefore, the variations in the charging state among the plurality of cells are kept. In addition, if this state is left as it is, there is a fear that variations increase.
From the past, in an assembled battery configured by connecting a plurality of battery cells in series, a voltage equalizer circuit (hereinafter, will be referred to as cell-balancing circuit) that equalizes the voltages of the battery cells is used for the purposes of using a battery's maximum capacity, preventing the occurrence of the battery variations due to deterioration, and the like. For the cell-balancing circuit, various configurations are known, for example, as shown in Patent Documents 1 and 2.
Patent Document 1: Japanese Patent Application Laid-open No. 2003-289629
Patent Document 2: Japanese Patent Application Laid-open No. HEI 11-176483